var xhr = new XMLHttpRequest();
xhr.open("GET", "/api/plugin/settings/api/get_bg", true);
xhr.onreadystatechange = function () {
    if (xhr.readyState == 4 && xhr.status == 200) {
        document.getElementById("win_app").setAttribute("style", JSON.parse(xhr.responseText)["data"])
    }
}
xhr.send();